1. Field of the Invention
The present invention relates to a CPP-GMR device for reading the magnetic field strength of a magnetic recording medium or the like as signals, a thin-film magnetic head comprising that CPP-GMR device, and a head gimbal assembly and a magnetic disk system comprising that thin-film magnetic head.
2. Explanation of the Prior Art
As the density of hard disks (HDDs) increases, there are growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has been widely used, which has a structure wherein a reproducing head having a read-only magneto-resistive effect device (MR device) and a recording head having a write-only induction type magnetic device are stacked together.
A magneto-resistive effect device of the type that operates on currents flowing parallel with the device plane—called the spin valve GMR device—is now widely used as the reproducing head. The spin valve GMR device is located between upper and lower shield layers, each formed of a soft magnetic metal material, via an insulating layer called a gap layer. The recording density of the device in the bit direction is determined by the spacing between the upper and the lower shield layer (reproducing gap spacing).
With recent increases in the recording density, there are rising demands for the reproducing head to have a narrower shield gap or track. A decreasing track width gives rise to a device height decrease, resulting in a device area decrease. A problem with the prior art structure is that a worsening of heat radiation efficiency due to the device area decrease places some limits on operating currents in view of reliability. To solve that problem, there has been a head structure proposed, wherein the first and the second shield film are connected electrically in series with the magneto-resistive effect device to dispense with any inter-shield insulating layer. Called the current-perpendicular-to-plane (CPP) structure, such structure is considered essential to achieve a recording density exceeding 200 Gbits/in2.
The general arrangement of the “spin valve type” CPP-GMR device here is briefly described. Part of the spin valve type CPP-GMR device has a multilayer structure comprising a ferromagnetic film (1) and a ferromagnetic film (2) separated off by an electroconductive, nonmagnetic intermediate layer. Referring specifically to the structure of part of a typical spin valve type CPP-GMR device, it has a multilayer structure comprising a lower electrode/antiferromagnetic film/ferromagnetic film (1)/nonmagnetic intermediate layer/ferromagnetic film (2)/upper electrode stacked together in order. In such a multilayer structure, the uppermost layer defines the upper electrode and the lowermost layer defines the lower electrode. When the magnetic field applied from outside is zero, the direction of magnetization of the ferro-magnetic film (1) that is one of the ferromagnetic films is fixed in the perpendicular direction to that of the ferromagnetic film (2).
The direction of magnetization of the ferromagnetic film (1) is fixed by providing the antiferromagnetic film adjacent to the ferromagnetic film (1) so that uni-directional anisotropic energy (or also called an exchange bias or a coupling magnetic field) is imparted to the ferromagnetic film (1) by way of exchange coupling. For this reason, the ferromagnetic film (1) is usually called a fixed magnetization (pinned) layer.
In such a CPP-GMR device, the smaller the sectional area of the device, the larger the resistance value grows, and the more the amount of resistance change becomes as well. In other words, that device has the feature of being fit for making track width narrower. Even with a decreasing device size, however, the resistance value of the device remains very low whenever it is made of an ordinary metal material alone. Diverse methods have thus been figured out to increase the amount of resistance change. As one typical method, a current-narrowing type CPP-GMR device has been proposed in the art (for instance, JP(A)'s 2002-208744 and 2006-54257). This CPP-GMR device structure has the feature of comprising an insulating layer interposed between the aforesaid ferromagnetic film (1) and the aforesaid ferromagnetic film (2) and a nonmagnetic intermediate layer including a current path extending through the insulating layer. The material of the nonmagnetic intermediate layer forming the current path extending through the aforesaid insulating layer contains at least one element selected from the group consisting of Cu, Au, and Ag. The use of such a current-narrowing effect makes it possible to reduce the proportion of parasitic resistance (for instance, the resistance of the aforesaid antiferromagnetic film) occurring for the reason of low resistance that is the feature and advantage of the CPP-GMR device by controlling the flow of sense currents. It is thus possible to decrease the proportion of parasitic resistance that does not essentially contribute to resistance changes, thereby improving the MR change ratio.
At an area of the current-narrowing CPP-GMR device having a low area resistivity (AR), for instance, at an area of AR=0.2 to 0.3Ω·μm2, however, the MR change ratio remains as low as 4 to 5%, a figure still inadequate for a possible practical head having a recording density exceeding 400 Gbpsi, and so further improvements in the MR change ratio are still in strong demand.
The situations being like such, an object of the present invention is to provide a CPP-GMR device with which further improvements in the MR change ratio at an area of low area resistivity AR are brought about and resistance to magnetic field is enhanced with high reliability as well, so that the coming-generation head having such a recording density as exceeds 400 Gbpsi could be practically achieved.